6-mercaptopurine (6-MP) is a synthetic analogue of natural purine bases. After absorption into the body, it is transformed into nucleotides which interfere with nucleic acid biosynthesis, especially in the active S phase. As such, it used to slow the growth of cancerous cells. 6-MP is indicated as a monotherapy and as part of combination therapies for treating acute lymphocytic leukemia in both adults and children (Physician's Desk Reference 57th Edition, 2003, page 1615-1618). 6-MP also exhibits immunosuppressive properties. While it is not officially indicated for diseases where treatment with immunosuppressive agents is beneficial, 6-MP has been widely used for several such conditions, especially for Crohn's disease and colitis.
6-MP is administered orally and has partial and variable absorption and bioavailability. Approximately 50% of an oral dose is absorbed. 6-MP is further subject to metabolism, especially by thiopurine methyltransferase.
The need for improving the therapeutic potential of 6-MP has been known for a long time. U.S. Pat. Nos. 4,443,435 and 5,120,740, among others, describe the preparation of prodrugs for 6-MP as ways of improving the use of this potent drug. Work of this sort continues, as is seen in U.S. Patent Application Publications 20040013728, 20030232760, and 20020013287. U.S. Pat. Nos. 6,680,302; 6,576,438; and 6,355,623 describe methods of improving the therapeutic outcome of 6-MP treatment in leukemia and in bowel diseases such as Crohn's disease or colitis by monitoring metabolites of the 6-MP and/or thiopurine methyltransferase activity and setting dosing based on the results. U.S. Pat. Nos. 6,692,771 and 6,680,068 and U.S. Patent Application Publications 20030077306 and 20020160049 describe emulsion formulations that may help the penetration of 6-MP into the body, while U.S. Pat. Nos. 6,602,521 and 6,372,254, and U.S. Patent Application Publications 20030133976 and 20020164371 describe drug delivery systems that might improve the therapeutics of 6-MP. None of these latter patents show data demonstrating improved bioavailability or therapeutic outcomes with 6-MP. The need still exists for formulations for improved delivery of 6-MP that improve the bioavailability thereof.
Standard 6-MP tablets (described in Physician's Desk Reference 57th Edition, 2003, page 1615-1618) reach full dissolution after about an hour under acidic dissolution conditions using a USP type II dissolution unit with paddles rotating at 50 rpm. 50% dissolution is reached at between 10 and 15 minutes. This rate of dissolution is not as fast as would be desirable. One method of improving the rate of dissolution of poorly soluble powders is to micronize them. In the case of 6-MP, micronization does little to improve the rate of dissolution of formulated tablets when compared to the standard formulation. The lack of improved rate of dissolution makes such tablets unlikely to show improved bioavailability when compared to the standard formulation. Further improvements to the formulation are clearly needed.